Titanate ceramics are widely used as constituting materials for electronic components such as capacitors, filters and transducers, since they have excellent characteristics such as insulation properties, ferroelectricity, piezoelectric properties, pyroelectricity and semiconducting properties. Such ceramics can be prepared by a solid state reaction method wherein raw materials comprising oxides or carbonates are mixed in a wet system, pulverized, dried and calcined. However, ceramics obtained by such a solid phase reaction method are in the form of aggregated or coarse grains and thus require a mechanical pulverization operation by e.g. a ball mill method. Accordingly, they have a drawback that impurities from the pulverization container or pulverization medium are likely to be included. Further, by such a pulverization operation, it is at best possible to obtain a powder of a micron order or a submicron order, and it is not possible to obtain fine particles (nano powder) having a particle diameter of less than the submicron order.
As a method for obtaining homogeneous fine ceramic particles having higher purity and a small particle diameter, a wet system preparation method such as a spray pyrolysis method, a coprecipitation method, a sol-gel method, an alkoxide method, an oxalate method or a hydrothermal synthesis method, has been proposed. However, even when a spray pyrolysis method, a coprecipitation method, a sol-gel method, an alkoxide method or an oxalate method is employed, a mechanical pulverization operation will be required in order to obtain a nano powder, whereby inclusion of impurities from the pulverization container or pulverization medium will be a problem like in the case of the solid state reaction method. Therefore, a case is limited wherein such a method can be used for the synthesis of fine ceramic particles.
In a hydrothermal synthesis method which requires no pulverization step, the synthesis is carried out under a high temperature and high pressure conditions, whereby it is not easy to set the conditions for the synthesis, and it is not easy to judge the terminal point of the synthesis. Accordingly, it is difficult to control the particle size and the particle size distribution of the desired product.
Further, fine ceramic particles synthesized by such a wet system preparation method, contain OH groups, etc. particularly on their edges and faces and thus tend to be inadequate from the viewpoint of crystallinity. Thus, none of the above methods was useful as a means to synthesize homogeneous fine ceramic particles having high purity and high dielectric characteristics, which can be used as a constituting material for electronic components.
Further, by the gas phase reaction method, it is difficult to obtain a material in a particle form, the productivity is low, the cost is high, and the apparatus tends to be of a large scale. Therefore, the gas phase reaction method is rarely employed as a means to synthesize fine particles of titanate ceramics.
On the other hand, U.S. Pat. No. 4,569,775 discloses a process for producing a magnetoplumbite type ferrite powder by a glass crystallization method wherein glass is heat-treated at a temperature of at least the glass transition point to precipitate ceramic crystals in a glass matrix, and then, the glass is dissolved and removed with a weak acid to separate only the crystallites. Such a glass crystallization method is effective as a means to synthesize high purity ceramic particles in a case where substances other than the crystallites can be completely removed and has a characteristic that control of the shape of the particles is thereby easy.
Bismuth titanate as one of ceramics for electronic components, has excellent characteristics such that the Curie point is high, it has a large remanence, and the temperature dependency of resonance frequency is low, and it is thus expected to be useful as a constituting material for semiconductor memories of the next generation. At present, a solid state reaction method is employed for the synthesis of bismuth titanate particles. However, recently, it is desired to develop a process for producing fine particles of bismuth titanate which have higher crystallinity, a small particle diameter and excellent dielectric characteristics.